1. Field
The present embodiments generally relate to the gasification of carbon based materials to produce syngas. More particularly, embodiments of the present invention relate to a carrier fluid for feed to transport gasification.
2. Description of the Related Art
Gasification is a high-temperature process usually conducted at elevated pressures to convert carbon-containing materials into carbon monoxide and hydrogen gas. Since this gas is often used for the synthesis of chemicals or synthetic hydrocarbon fuels, the product gas is often referred to as “synthesis gas” or “syngas.” Syngas can be used as a fuel to generate electricity or steam and as a source of hydrogen. Syngas can also be used to produce a wide range of products, such as fuels, chemicals, fertilizers, and industrial gases.
Typical feeds to gasification include petroleum-based materials that are neat or residues of processing materials, such as heavy crude oil, bitumen, tar sands, kerogen, oil shale, coke, and other high-sulfur and/or high metal-containing residues; gases; and various carbonaceous and waste materials. The feedstock is reacted in the gasifier in a reducing (oxygen-starved) atmosphere at high temperature and (usually) high pressure. The resulting syngas typically contains about 85 percent of the feed carbon content as carbon monoxide, with the balance being a mixture of carbon dioxide and methane.
There are three basic types of gasifiers: fixed bed, fluidized bed, and entrained flow. The fixed bed gasifier is operated at a relatively low temperature of about 425° C. (797° F.) to about 600° C. (1,112° F.) and requires less oxygen compared to the other two types of gasifiers; however, the product syngas contains substantial amounts of unconverted methane, and by-product tars and oils. The fluidized bed gasifier operates at more moderate temperatures of about 900° C. (1,652° F.) to about 1,050° C. (1,922° F.) and requires a greater amount of oxygen than a comparable fixed bed gasifier. While the synthesis gas from a fluidized bed gasifier is of higher purity, the carbon conversion is lower than a comparable entrained flow gasifier which operates at a much higher temperatures of about 1,250° C. (2,282° F.) to about 1,600° C. (2,912° F.) and requires significantly higher energy input.
The high temperature in entrained gasifiers and in the lower zones of certain fixed bed gasifiers converts the inorganic materials in the feed into a molten vitrified material which solidifies when removed from the gasifier, producing a material resembling coarse sand and generally referred to as slag. Fluid bed gasifiers produce dry ash which is not vitrified but only consolidated or agglomerated. Depending on the gasifier, it is desirable to remove ash at lower temperatures (non-slagging gasifiers) or as a low viscosity liquid at high temperatures (slagging gasifiers). This inert slag or ash has a variety of uses in the construction and building industries.
The feedstock is typically transported into a gasifier with a carrier fluid, such as air or nitrogen. The carrier fluid is typically pre-heated to provide sensible heat to the gasification process. Air, however, is limited as a source of heat to the gasifier because the temperature of the air needs to be kept fairly low to avoid reactions between the feedstock and oxygen. Moreover, the oxygen combustion with the volatiles in the feedstock increases the propensity for feedstock/ash sintering, and carbon dioxide production, all of which reduce the heating value of the syngas. Nitrogen is inert to the gasification process, but also reduces the heating value of the syngas since nitrogen has no heating value itself. There is a need, therefore, for a process for gasification having a carrier fluid that does not significantly reduce the heating value of the product gas.